The Fermi Gamma-Ray Space Telescope helps scientists see traces of stars that sparked just 500 million years after the big bang.

This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma rays with energies above 10 billion electron volts (GeV) detected by Fermi. The plane of our Milky Way galaxy runs along the middle of the plot. Credit: NASA/DOE/Fermi LAT Collaboration.

The universe started with a blackout. After the extraordinary burst of the big bang, scientists say, it was dark for hundreds of millions of years as scattered particles cooled. Then there was light, as the first stars condensed out of gaseous cosmic clouds. Now a team of international astrophysicists has detected traces of these very first stars using the Fermi Gamma-Ray Space Telescope, according to a paper published in Science.

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All the light created by all the stars throughout history combines together in a glow astrophysicists call extragalactic background light, or EBL. The research team used the Fermi telescope to separate out the light of the earliest stars, sparked 500 million years after the big bang, which itself happened about 13.7 billion years ago.

To see it, they aimed the telescope at distant blazars. Aside from having the coolest name of all space objects, blazars are useful because these giant black holes give off huge "blazes" of gamma rays when matter is caught in their grasp, sucked in, and turned to energy. By looking at how bright the blazars appear through the background light, the researchers could tell how much EBL is between the black holes and the telescope (and thus how old they are).

After blasting out of a blazar, the gamma rays travel through space. As they travel, inevitably some of the gamma rays collide with a photon of starlight and are split into electrons and positrons, ending their journey. The rest of the gamma rays arrive at Fermi unchanged. For a blazar that is very far away, fewer gamma rays make it to our telescope.

Using several measurements from blazars at different distances, the scientists established a baseline for how thick or thin the cosmic light glow has been as far back as 500 million years after the Big Bang. This is the first time scientists have directly measured the EBL.

Their findings seem to suggest that the first stars formed more slowly than anticipated. They can't yet say exactly when the first stars appeared, but they now have a proven method for looking into the dark period before stars were born.

In the future, the researchers plan to look even farther back into the past. Co-author Marco Ajello, of Stanford's Kavli Institute for Particle Physics and Cosmology, said at a press conference that the James Webb Space Telescope, NASA's successor to Hubble, should be orbiting the sun by 2018 and looking directly at the earliest galaxies.